Bifacial tandem solar cells

ABSTRACT

A method of fabricating on a semiconductor substrate bifacial tandem solar cells with semiconductor subcells having a lower bandgap than the substrate bandgap on one side of the substrate and with subcells having a higher bandgap than the substrate on the other including, first, growing a lower bandgap subcell on one substrate side that uses only the same periodic table group V material in the dislocation-reducing grading layers and bottom subcells as is present in the substrate and after the initial growth is complete and then flipping the substrate and growing the higher bandgap subcells on the opposite substrate side which can be of different group V material.

FIELD OF THE INVENTION

This invention relates to a method of fabricating on a semiconductorsubstrate bifacial tandem solar cells with a lower bandgap than thesubstrate bandgap on one side and with a higher bandgap than thesubstrate on the other.

BACKGROUND OF THE INVENTION

Tandem solar cells, also known as multijunction, cascade, ormulti-bandgap solar cells, contain two or more subcells, with eachsubcell containing a PN junction designed to convert light intoelectrical power from a different part of the solar spectrum. Subcellsare made from various semiconductor materials. In the final solar cell,the subcells are vertically stacked, in order, from the subcell with thehighest bandgap in the uppermost position to the subcell with the lowestbandgap in the lowest position in the stack. The highest bandgap subcellreceives the incident sunlight first, absorbing mostly short wavelengthlight, while its material allows longer wavelength light to pass throughvirtually unabsorbed. Each subsequent subcell in the tandem stack thenabsorbs some remaining light, passing on the longer wavelengths of thelight. The purpose of these multiple subcells, each of a differentbandgap, is to more efficiently utilize the solar spectrum and increasethe solar to electrical power conversion efficiency compared to a singlejunction solar cell.

The semiconductor materials used for the substrate or support substrateand the various stacked subcells (e.g. top, middle, bottom cell in athree junction tandem cell) epitaxially grown on the substrate shouldhave nearly the same (e.g. within a few thousand parts per million)semiconductor crystalline lattice constant (e.g. 5.65 angstroms forgallium arsenide based cells) to avoid crystal defects (e.g.dislocations) which may form to relieve the mechanical strain caused ifthere is lattice mismatch between subcells. These dislocation defectsoften act as minority carrier recombination sites, which degrade thepower conversion efficiency of the solar cell. The choices of materialsfor the support substrate and the various component subcells (top,middle, etc.) of the tandem are thus limited to those closelylattice-matched with each other. Useful tandem cells, known asmetamorphic cells, can be made despite the presence of significantlattice mismatch, if a greater benefit can be achieved (e.g. freedom tomatch bandgaps of subcells to maximize and equalize the electricalcurrent generated in each subcell) than that given up by theintroduction of the dislocations.

One approach to this problem is to use bifacial, tandem solar cellswhere the subcells can be placed on both sides of a substrate to isolatethe effects of lattice mismatch to cells on one side of the substrate.U.S. Patent Publication US2006/0162768 A1, by Wanlass et al. covers thegeneral case of a transparent semiconducting substrate with subcellsgrown on each side. In comparison with Wanlass, the inventions claimedhere are related to particular non-obvious process sequences used togrow the subcells on each side of the substrate. In US 2005/0056312 A1,Young et al. describes subcells on opposite sides of an insulating glasssubstrate and in U.S. Pat. No. 4,289,920 Hovel describes subcells onopposite sides of an insulating ceramic substrate. Both of thesepatents, although describing a bi-facial growth, do not use epitaxialgrowth on a semiconducting substrate on which our particular inventionclaims are based.

Currently inverted metamorphic processes are being used to make highefficiency tandem solar cells but those processes involve substratebonding and epitaxial lift-off which are lower yield when used overlarge substrate areas. “40.8% Efficient Inverted Triple-Junction SolarCell With Two Independently Metamorphic Junctions” by J. F. Geisz etal., Applied Physics Letters 93, 123505 (2008).

SUMMARY OF THE INVENTION

In accordance with various aspects of the subject invention in at leastone embodiment the invention presents an improved method of fabricatingbifacial tandem solar cells with lower bandgap subcells on one side ofthe substrate and high bandgap subcells on the other which is easier,simpler and more reliable even over larger substrate areas than presentinverted metamorphic processes currently used to fabricate similartandem cells.

The subject invention results from the realization that, in part, animproved method of fabricating on a semiconductor substrate bifacialtandem solar cells with a lower bandgap than the substrate bandgap onone side and with a higher bandgap than the substrate on the other invarious aspects can be achieved either by growing a lower bandgapsubcell or subcells whose material composition is similar to thesubstrate and then flipping the substrate and growing higher bandgapsubcells, or, by growing the higher bandgap subcells first on either asubstrate whose backside has a protective film, or, if not, then etchingthe substrate surface before proceeding with lower bandgap subcellgrowths.

The subject invention, however, in other embodiments, need not achieveall these objectives and the claims hereof should not be limited tostructures or methods capable of achieving these objectives.

This invention features a method of fabricating on a semiconductorsubstrate bifacial tandem solar cells with semiconductor subcells havinga lower bandgap than the substrate bandgap on one side of the substrateand with subcells having a higher bandgap than the substrate on theother. First a lower bandgap subcell is grown on one substrate side thatuses only the same periodic table group V material in thedislocation-reducing grading layers and bottom subcells as is present inthe substrate and after the initial growth is complete then flipping thesubstrate and growing the higher bandgap subcells on the oppositesubstrate side which can be of different group V material.

In preferred embodiments after the lower bandgap subcells are grown, thelower bandgap subcell epilayers may be capped with a suitable protectivelayer before continuing on to the growth of higher bandgap subcells onthe opposite substrate side. The protective layer may include siliconnitride. The substrate may be a lightly doped (less than 5×10¹⁷ cm⁻³substrate)N-type GaAs or InP substrate. The grading layers maybe one ofInx(Al_(y)Ga_(1-y))_(1-x) P and In_(x)(Al_(y)Ga_(1-y))_(1-x)As. Thebottom subcells may be made of In_(x)Ga_(1-x)As. The first higherbandgap subcell may include a N-on-P cell of In_(x)Ga_(1-x)As or GaAs.The second higher bandgap subcell may be made of In_(x)Ga_(1-x)P orIn_(x)(Al_(y)Ga_(1-y))_(1-x)P. A bottom contact cap may includeadditional thickness so that a portion of the bottom cap can be etchedaway to remove any area damaged during epitaxial growth of the upperhigher bandgap subcells.

The invention also features a method of fabricating on a semiconductorsubstrate bifacial tandem solar cells with subcells having a lowerbandgap than the substrate bandgap on one side of the substrate and withsubcells having a higher bandgap than the substrate on the other sidemade by first growing higher bandgap subcells that can have differinggroup V materials (e.g. GaAs middle and InGaP top cells) on one side ofthe GaAs substrate including applying a protective layer to the uppersubcell epilayers, etching or polishing the back substrate surface toremove damaged substrate material, growing an epitaxial conditioningbuffer of the same material as the substrate on the etched surface. andepitaxially growing the grading layers and at least one bottom subcell.

In preferred embodiments the substrate may be a lightly doped (less than5×10¹⁷ cm⁻³ substrate)N-type GaAs or InP substrate. The grading layersmay be one of In_(x)(Al_(y)Ga_(1-y))_(1-x) P andIn_(x)(Al_(y)Ga_(1-y))_(1-x)As. The bottom subcells may be made ofIn_(x)Ga_(1-x)As. The first higher bandgap subcell may include a N-on-Pcell of In_(x)Ga_(1-x)As or GaAs. The second higher bandgap subcell maybe made of In_(x)Ga_(1-x)P or In_(x)(Al_(y)Ga_(1-y))_(1-x)P.

This invention also features a method of fabricating on a semiconductorsubstrate bifacial tandem solar cells with subcells having a lowerbandgap than the substrate bandgap on one side of the substrate and withsubcells having a higher bandgap than the substrate on the other sidemade by first coating the substrate backside with a protective layerthen growing higher bandgap subcells that can have differing group Vmaterials on one side of the GaAs substrate including applying aprotective layer if needed to the upper subcell epilayers, removing theprotective film on the substrate backside, growing an epitaxialconditioning buffer of the same material, and then epitaxially growingthe grading layers and at least one bottom subcell.

In preferred embodiments the substrate may be a lightly doped (less than5×10¹⁷ cm⁻³ substrate)N-type GaAs or InP substrate. The grading layersmay be one of In_(x)(Al_(y)Ga_(1-y))_(1-x) P andIn_(x)(Al_(y)Ga_(1-y))_(1-x)As. The bottom subcells may be made ofIn_(x)Ga_(1-x)As. The first higher bandgap subcell may include a N-on-Pcell of In_(x)Ga_(1-x)As or GaAs. The second higher bandgap subcell maybe made of In_(x)Ga_(1-x)P or In_(x)(Al_(y)Ga_(1-y))_(1-x)P.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled inthe art from the following description of a preferred embodiment and theaccompanying drawings, in which:

FIG. 1 is a schematic diagram of a low bandgap tandem subcell grown on ahigher bandgap conducting, transparent semiconductor substrate;

FIG. 2 illustrates the fabrication of the preferred embodiment of thebifacial tandem cell by showing a middle cell grown on the substrateside opposite the bottom cell following flipping the substrate afterbottom cell epitaxial growth;

FIG. 3 shows the next step of the first preferred embodiment, in which atop cell, with a higher bandgap than any of the previous subcells, isfinally grown;

FIG. 4 is a flow chart showing the steps and layers according to onedetailed embodiment of the method and also may be viewed as a detailedcross-section schematic view of a solar cell made according to theinvention;

FIG. 5 is a schematic diagram of a solar cell in accordance with anotherembodiment of the invention with a substrate having a middle and thentop subcell grown on its front side;

FIG. 6 is a schematic diagram of the solar cell of FIG. 5 with thesubstrate flipped with the lower bandgap bottom subcell grown on theopposite side; and

FIG. 7 is a schematic diagram of the tandem cell of FIG. 6 simplyflipped again so the top cell receives incident light first.

DETAILED DESCRIPTION OF THE INVENTION

Aside from the preferred embodiment or embodiments disclosed below, thisinvention is capable of other embodiments and of being practiced orbeing carried out in various ways. In particular, the invention isapplicable to either N-on-P or P-on-N tandem structures. Thus, it is tobe understood that the invention is not limited in its application tothe details of construction and the arrangements of components set forthin the following description or illustrated in the drawings. If only oneembodiment is described herein, the claims hereof are not to be limitedto that embodiment. Moreover, the claims hereof are not to be readrestrictively unless there is clear and convincing evidence manifestinga certain exclusion, restriction, or disclaimer.

It has been generally accepted by persons skilled in the art that thedesired configuration for monolithic multijunction tandem devices isbest achieved by lattice matching the material for all componentsubcells (e.g. top, middle, bottom, etc.) in the tandem cell to thesubstrate or substrate material. Mismatches in the lattice constantscreate dislocation defects in the crystal lattice which can act asminority-carrier recombination centers, causing the loss ofphotogenerated minority carriers and thus significantly degrading thephotovoltaic efficiency of the device.

However, the common design rule that the materials used in the solarcell need to be lattice matched is not the only constraint on the tandemcell design. Another important constraint is that just the right amountof light be absorbed in each subcell so that the photocurrent generatedin each subcell is very similar to all other subcells. This secondcurrent-matching constraint allows the subcells to be connected togetherin series internally by means of a tunnel junction between subcells, andallows the final user of the solar cell to only have to connect to twoterminals, one at the top and one at the bottom of the cell. In orderfor the current in each series-connected subcell (e.g. top, middle,bottom, etc.) in the tandem structure to be the same (e.g.current-matched), only certain bandgap combinations can be used so thateach cell absorbs just the right amount of light.

FIG. 1 is a schematic diagram of a low bandgap tandem subcell 210 grownon a higher bandgap conducting, transparent semiconductor substrate 212.In the preferred embodiment, this bottom subcell 210 will have anoptimum bandgap which will likely not be lattice-matched to thesubstrate. Dislocation defects are created to relieve stress caused bythe lattice mismatch between the atomic spacing of the subcell andsubstrate material. The invention will limit their influence to subcellson one side of the substrate.

FIG. 2 continues to illustrate the fabrication of the preferredembodiment of the bifacial tandem cell by showing a middle cell 214grown on the substrate 212 side opposite the bottom cell 210 after thesubstrate is flipped at the epitaxial growth reactor. One importantadvantage of this embodiment is that no surface preparation is neededafter the bottom subcell 210 growth to grow cells on this opposite side.This middle cell 214 has a higher bandgap than the substrate 212, whichin turn has a higher bandgap than the bottom cell 210 to allow light totransit through the substrate to the bottom cell. An option in theprocess after the lower bandgap subcell 210 growth is to deposit asuitable (e.g. silicon nitride) protective layer that can survive thehigh temperature epigrowth and is thermally expansion matched to thesubstrate so that the contact cap of the bottom cells is not degradedduring top cell growth.

FIG. 3 shows the final figure of the first preferred embodiment, inwhich a top cell 216, with a higher bandgap than any of the previoussubcells or substrates, is finally grown.

One specific embodiment includes a three junction tandem cell suitablefor one-sun or concentrator space or terrestrial cells, in which thelattice matched top and middle cells are grown on one side of thesubstrate and the lattice mismatched bottom cell is grown on theopposite side of the substrate as shown in FIG. 4.

The invention contemplates at least two approaches to creating thestructure of FIG. 4. One is fabricating bifacial cells by firstepitaxially growing a cell or cells with a lower bandgap than thesubstrate bandgap on one side of a semiconducting substrate, and thenflipping the substrate, and growing one or more cells with a higherbandgap on the opposite side. The second is fabricating bifacial cellsby first epitaxially growing a cell or cells with a higher bandgap thanthe substrate bandgap on one side of a semiconducting substrate, andthen flipping the substrate and growing one or more cells with a lowerbandgap on the opposite side. Both approaches use a number of standardprocesses/sequences but in the special order according to thisinvention.

The first and preferred approach includes growing subcell(s) withbandgaps lower than substrate on one side of the substrate, flipping thesubstrate and growing subcells with bandgaps equal to or higher than thesubstrate on the opposite side of the substrate.

Substrate 27, FIG. 4, is N-type and lightly doped (generally <5×10¹⁷cm⁻³) in order to limit free carrier absorption and allow almost all ofthe infrared light traveling through the substrate into the lowerbandgap backside subcells. An embodiment with one backside subcell isshown in FIG. 4, layers 28-42. Substrates are generally polished on bothfront and back to present a smooth growth surface, although acceptablegrowths can be performed on just an etched but not polished backsidesurface.

Buffer layer 28, generally of the same material and doping type as thesubstrate, is first grown to “condition” the back surface of thesubstrate 27 and improve the quality of subsequent epilayer growth.Although inclusion of this layer is preferred, acceptable growth canoccur if this layer is omitted, especially on a polished substrate 27starting surface.

A series of epitaxial “grading” layers 29-37 are grown next, again ofthe same N doping type as the substrate. The initial grading layer 29has a lattice constant near that of the substrate (e.g. 5.65 angstromsif a GaAs substrate). The final grading layer 37 has a differentlattice-constant that is determined by the bandgap needed by theadjacent bottom subcell to achieve an acceptable matched photocurrentwith the other planned subcells of the tandem. Each grading layer has anintermediate lattice constant between the initial and final values. Inone embodiment, the linear step grade, the layers are made of equalthickness and each lattice constant (or equivalently each materialcomposition) of the intermediate grading layers 29-37 are adjusted tohave a uniform increment from the initial to the final value. Othertypes of grades with non-linear step thicknesses and compositions can beused.

In a preferred embodiment using an N-type GaAs substrate, materials usedfor the grading layer include In_(x)(Al_(y)Ga_(1-y))_(1-x)P andIn_(x)(Al_(y)Ga_(1-y))_(1-x)As. Compositions from materials such asthese are preferred since the compositions of the grading layers can beadjusted to have a higher bandgap than the GaAs substrate, minimizingabsorption loss of the light emerging from the substrate before thelight reaches the lower bandgap cells.

In_(x)(Al_(y)Ga_(1-y))_(1-x)As is preferred overIn_(x)(Al_(y)Ga_(1-y))_(1-x)P for grading layers when growing the bottomcells because the opposite side of the GaAs substrate (upper side oflayer 27) is essentially undamaged during the growth of the InAlGaAsgrading layers and bottom cell. After flipping the substrate, this uppersurface is suitable for immediate growth of the upper higher bandgapsubcells and tunnel junctions (layers 6-26, GaAs middle and InGaP topcells and tunnel junctions) as long as an arsine atmosphere was used forthe bottom cell growth e.g. an In_(x)(Al_(y)Ga_(1-y))_(1-x)As grade andInGaAs subcell (layers 38-42). Use of any phosphorous (e.g. growth in aphosphine atmosphere such as for an In_(x)(Al_(y)Ga_(1-y))_(1-x)P grade)tends to damage the opposite GaAs substrate surface which will thenrequire additional effort to prepare for the growth of the upper cells.

After growth of the buffer and grading layers 28-37, the bottom cell38-41 is grown. Typical thicknesses, doping and compositions are shownfor these window, emitter, base, and back surface field, layers 38-41,in FIG. 4, but variation is possible. The final cap layer 42 serves twopurposes. Traditionally, this layer is heavily doped and of low bandgapto make a low resistance ohmic contact to the metal layers 43 to 45 onthe backside of the substrate. In addition, this layer is thicker thannormal (between 500 and 2000 nm). The purpose of the thick cap growth isto have enough material so that a portion of the cap layer can be etchedaway after growth during subsequent processing to remove a damaged layerof material that resulted when the InGaP top cell is grown on theopposite side of the substrate.

In one variation of this growth technique, as an alternative to the capetch, the substrate is removed from the epitaxial reactor and the lastlayer grown (the cap) is protected with a 50-500 nm silicon nitride orother suitable protective film by plasma enhanced chemical vapordeposition or other deposition technique. The purpose of this temporaryprotective layer (it is removed during subsequent cell processing) is toprevent degradation of the bottom subcell surface during growth of theupper high bandgap InGaP based top subcells in a phosphine atmosphere.The protective film should be relatively dense to serve as a gooddiffusion barrier to phosphorous and dopant atoms and should bethermally expansion matched to the bottom subcell material.

After epigrowth of layers 28-42 and cooling, the substrate is flipped inthe epitaxial reactor glovebox so that growth of the tunnel junctionsand higher bandgap subcells layers 6-26 can proceed on the oppositesubstrate layer 27 side. A buffer layer 26 may be first grown on theflipped substrate surface to help condition the substrate for subsequentgrowth.

Next, the first tunnel junction, layers 21-25, is then grown using theN-on-P cell geometry with a low infrared light absorption N-type GaAssubstrate. A Te dopant flush step 25 saturates the reactor lines andsurfaces so that the Te is more readily incorporated into the growingfilm interface instead of being adsorbed elsewhere in the reactor duringthis growth step. In this first tunnel junction, a highly doped N GaAslayer 24 can be grown appreciably thicker than indicated in FIG. 4 sincethere is little light remaining at this point that the GaAs can absorb.The reactor is then flushed of the Te dopant gas 23 and a very thinundoped GaAs layer 22 is grown to help make an abrupt interface at a PNjunction with a high bandgap transparent carbon-doped highly P-typeAlGaAs layer (21) grown at low V-III ratio.

A standard N-on-P GaAs cell, layers 17-20, is grown. There are otherpossible variations of doping and thickness and composition of thewindow, emitter, base, and back surface field layers of this middlesubcell. In general, the window of this subcell, layer 21, can be madeto have a lower bandgap than a window in a stand-alone single-junctionGaAs subcell since the upper InGaP subcell, layers 6-11, absorbs much ofthe higher energy photons. Advantages of a lower bandgap N-type AlGaAswindow are slightly lower interface recombination due to slightly lessmismatch and slightly lower series resistance due to a lower electronbarrier at the interface.

The second tunnel junction, layers 12-16, is then grown using the N-on-Pcell geometry. A Te dopant flush step 16 again saturates the reactorlines and surfaces. In this second tunnel junction, the highly doped NGaAs layer 15 must be grown as thin as possible since it absorbs somelight that the underlying GaAs middle subcell 17-20 can absorb. Thereactor is then again flushed of the Te dopant gas 14 and a very thinundoped GaAs layer 13 is grown to help make an abrupt interface at a PNjunction with a high bandgap transparent carbon-doped highly P-typeAlGaAs layer 12 grown at low V-III ratio.

An advantage of this bottom-subcell-first growth approach is that bothtunnel junctions on the front side, whose performance (series resistanceand peak current) is sensitive to dopant diffusion, experience lessdopant diffusion than in the alternative top-subcell first growthapproach to be described later since the tunnel junctions are notsubject to additional high temperature annealing from the bottom subcellgrowth.

A standard N-on-P InGaP or InAlGaP cell, layers 6-11 is grown. There aremany possible variations of doping and thickness and composition of thewindow, emitter, base, and back surface field layers of this upper topsubcell. Of particular note, when using Te dopant for the N GaAs caplayer 6, including a dopant spacer layer 7 with a lower doping level ofsilicon, improves the cell open-circuit voltage and photocurrentcollection from the emitter layer 9. The InAlP window 8 serves as both aminority carrier mirror and a selective wet etch stop to remove the GaAscap layers 6-7 that are not under a metal gridline 3-5.

Substrate processing for the these cells proceeds as follows. If theoptional silicon nitride protection layer was used, it is etched away inbuffered HF acid. The substrate front side (upper and middle subcells)are protected with photoresist. Bottom cap 42 can be made additionallythick so that about half the bottom cap 42 is removed in a timed etch toremove most of the material damaged by exposure to phosphine during thetop subcell growth. The bottom metal contact stack 43-45 is evaporatedover the full area of the back subcell surface. Holes in photoresist formetal gridlines are photolithographically patterned and the top metalstack 3-5 is evaporated and then defined by a metal liftoff process.Mesas are then photolithographically patterned to protect the cell areaand a wet etch removes excess material down to the substrate. The metalon the substrate backside protects the back epi during this etch. Thismesa step may be unnecessary. There seems to be little performancedegradation in a concentrator solar cell application if it is cutdirectly through the upper epi versus in a street around the cellperiphery. The top cap is selectively etched using the metal gridlinesas a self-aligned mask and the InAlP window as an etch stop for a citricacid-based wet etch. Finally, an AR coating 1-2 is then deposited on thecells immediately after the cap etch to minimize oxidation of the InAlPwindow in air. Openings are photolithographically made over the contactarea busbars. The substrates are then diced into individual cells.

The second approach includes growing subcells with bandgaps equal to orhigher than the substrate on one side of substrate, flipping thesubstrate, and growing subcells with bandgaps lower than substrate onthe opposite side of the substrate.

Substrate layer 27 is N-type and lightly doped (generally <5×10¹⁷ cm⁻³)in order to limit free carrier absorption and allow almost all of theinfrared light traveling through the substrate into the lower bandgapbackside subcells: an embodiment with one backside subcell is shown inFIG. 4, layers 28-42. Substrates are generally polished on both frontand back to present a smooth growth surface, although acceptable growthscan be performed on just an etched but unpolished backside surface.

In one variation of this growth technique, before bringing substrates tothe epitaxial reactor, the backside substrate surface is protected witha 50-500 nm silicon nitride or other suitable protective film by plasmaenhanced chemical vapor deposition or other deposition technique. Thepurpose of this temporary protective layer (it is removed duringsubsequent processing) is to prevent degradation of the back GaAssubstrate surface during growth of the upper high bandgap InGaP basedtop subcells in a phosphine atmosphere. The protective film should berelatively dense to serve as a good diffusion barrier to phosphorous anddopant atoms and should be thermally expansion matched to the bottomsubcell material. A buffer layer 26 is first grown on the substratesurface to help condition the substrate for subsequent growth.

The first tunnel junction, layers 21-25 is then grown using the N-on-Pcell geometry with a low infrared light absorption N-type GaAssubstrate. A Te dopant flush step 25 saturates the reactor lines andsurfaces so that the Te is more readily incorporated into the growingfilm interface instead of being adsorbed elsewhere in the reactor duringthis growth step. In this first tunnel junction, a highly doped N GaAslayer (24) can be grown appreciably thicker than indicated in FIG. 4since there is little light remaining at this point that the GaAs canabsorb. The reactor is then flushed of the Te dopant gas 23 and a verythin undoped GaAs layer 22 is grown to help make an abrupt interface ata PN junction with a high bandgap transparent carbon-doped highly P-typeAIGaAs layer 21 grown at low ratio.

A standard N-on-P GaAs subcell (layers 17-20) is grown. There are manypossible variations of doping and thickness and composition of thewindow, emitter, base, and back surface field layers of this middlesubcell. In general, the window of this subcell layer 21 can be made tohave a lower bandgap than a window in a stand-alone single-junction GaAscell since the upper InGaP subcell, layers 6-11 absorbs much of thehigher energy photons. Advantages of a lower bandgap N-type AlGaAswindow are slightly lower interface recombination due to slightly lessmismatch and slightly lower series resistance due to a lower electronbarrier at the interface.

The second tunnel junction, layers 12-16 is then grown using the N-on-Pcell geometry. A Te dopant flush step 16 again saturates the reactorlines and surfaces. In this second tunnel junction, the highly doped NGaAs layer 15 must be grown as thin as possible since it absorbs somelight that the underlying GaAs middle subcell 17-20 can absorb. Thereactor is then flushed of the Te dopant gas 14 and a very thin undopedGaAs layer 13 is grown to help make an abrupt interface at a PN junctionwith a high bandgap transparent carbon-doped highly P-type AlGaAs layer12 grown at low V-III ratio.

A standard N-on-P InGaP or InAlGaP subcell, layers 6-11 is grown. Thereare other possible variations of doping and thickness and composition ofthe window, emitter, base, and back surface field layers of this uppertop subcell. When using Te dopant for the N GaAs cap layer 6, includinga dopant spacer layer 7 with a lower doping level of silicon, the cellopen-circuit voltage and photocurrent collection from the emitter layer9 is improved. The InAlP window 8 serves as both a minority carriermirror and a selective wet etch stop to remove the GaAs cap layers 6-7that are not under a metal gridline 3-5.

If the optional silicon nitride protection layer was used on the GaAssubstrate back surface, it is now etched away in buffered HF acid afterthe front side epitaxial growth. If the SiN was not used, the backsideGaAs substrate surface is contaminated with a residue from the InGaPsubcell growth. The front side of the substrate is now protected withresist and a phosphoric acid based etch is used to remove 8-12 um ofGaAs substrate material, leaving a rough etched surface. A buffer layer28, generally of the same material and doping type as the substrate, isfirst grown to “condition” the back surface of the substrate 27 andimprove the quality of subsequent epilayer growth. Thicker buffer layersof 500 nm of more are important to condition the back substrate forsubsequent growth on rough surfaces.

A series of epitaxial “grading” layers are next grown, of the same Ndoping type as the substrate. The initial grading layer 29 has a latticeconstant near that of the substrate e.g. 5.65 angstroms if a GaAssubstrate. The final grading layer 37 has a different lattice-constantthat is determined by the bandgap needed by the bottom cell to achievean acceptable matched photocurrent with the other planned subcells ofthe tandem. Each grading layer has an intermediate lattice constantbetween the initial and final values. In one embodiment, the linear stepgrade, the layers are made of equal thickness and each lattice constant(or equivalently each material composition) of the intermediate gradinglayers 29-37 are adjusted to have a constant increment from the initialto the final value. Other types of grades with non-linear stepthicknesses and compositions can be used.

In a preferred embodiment using an N-type GaAs substrate, materials usedfor the grading layer include In_(x)(Al_(y)Ga_(1-y))_(1-x)P andIn_(x)(Al_(y)Ga_(1-y))_(1-x)As. Compositions from materials such asthese are preferred since the compositions of the grading layers can beadjusted to have a higher bandgap than the GaAs substrate, minimizingabsorption loss of the light emerging from the substrate before thelight reaches the lower bandgap subcells.

In_(x)(Al_(y)Ga_(1-y))_(1-x)As is preferred versusIn_(x)(Al_(y)Ga_(1-y))_(1-x)P for grading layers when growing the bottomsubcells because the topmost epilayers of the upper subcell should beessentially undamaged during the bottom subcell growth as long as anarsine atmosphere was used for the bottom subcell growth e.g. anIn_(x)(Al_(y)Ga_(1-y))_(1-x)As grade and InGaAs layers 38-42. Use of anyphosphorous e.g. growth in a phosphine atmosphere such as for anIn_(x)(Al_(y)Ga_(1-y))_(1-x)P grade, would damage the upper epilayers ofthe topmost subcell on the other side of the substrate. If such a gradeis used, additional protection e.g. a SiN protective layer or a thickcap, may be used on the top cell cap layers 6 and 7 in FIG. 4.

After growth of the buffer and grading layers 28-37, the bottom subcellis grown, layers 38-41. Typical thicknesses, dopings and compositionsare shown for these window, emitter, base, and back surface field andcap layers 38-42 in FIG. 4, but variation is possible. The final caplayer 42 no longer serves two purposes and exists solely to make a lowresistance ohmic contact to the metal, layers 43 to 45, on the backsideof the substrate.

Substrate processing for these cells proceeds as follows. The substratefront side (upper and middle subcells) are protected with photoresist.The bottom metal contact stack 43-45 is evaporated over the full area ofthe back subcell surface. Holes in photoresist for metal gridlines arephotolithographically patterned and the top metal stack 3-5 isevaporated and then defined by a metal liftoff process. Mesas are thenphotolithographically patterned to protect the subcell area and a wetetch removes excess material down to the substrate. The metal on thesubstrate backside protects the back epi during this etch. This mesastep may be unnecessary. There seems to be little performancedegradation in a concentrator solar cell application if cut directlythrough the upper epi versus in a street around the subcell periphery.The top cap is selectively etched using the metal gridlines as aself-aligned mask and the InAlP window as an etch stop for a citricacid-based wet etch. An AR coating 1-2 is then deposited on the cellsimmediately after the cap etch to minimize oxidation of the InAlP windowin air. Openings are photolithographically made over the contact areabusbars. The substrate is then diced into individual cells.

In this approach substrate bowing issues are greatly reduced. There isno substrate bowing during growth of the upper InGaP based and GaAsbased subcells on the GaAs substrate. Bowing from the strain of the ˜2%lattice mismatched bottom subcells is reduced if the cells do not needto be thermally cycled back up to cell growth temperatures. Thinnersubstrates (less infrared transmission loss) can be used. In addition,the bottom cell is not degraded after it is grown by the InGaP based topcell growth. With the bottom-subcell-last growth approach the substratesneed to be removed from the reactor area after top subcell growth toeither remove a SiN protection layer or to wet etch the back surface toremove top subcell growth residue. Bottom cells grown on etched surfacesmay have slightly less performance than those grown on polishedsurfaces. However, the difference is small. Tunnel junctions on thefront side will see additional 2 hours of annealing at the bottomsubcell growth temperature, roughly 600 C. Although this may degrade bya factor of two or so in peak current and resistance, the performance isstill acceptable. The GaAs top subcell cap layer doping is lowered dueto additional diffusion during the bottom subcell growth, resulting insomewhat higher contact resistance if non-alloyed contacts are used.Traditional alloyed AuGeNi-based contacts which depend less on tunnelingmay be used. Bottom and top have been used to simplify explanation butit not limiting of the concept.

A more schematic view of this second approach is shown in FIGS. 5-7. Oneimportant advantage provided by the subject invention is that, forexample, if two subcells 110 and 112, FIG. 5 such as In(49%)Ga(51%)P andGaAs are epitaxially grown lattice-matched and current-matched on oneside of substrate 114, the substrate can be flipped, FIG. 6, and anadditional subcell 116 (one or more) can be grown on the opposite sideof the substrate in which the tandem cell designer can weigh the benefitof using material (e.g. compositions of InGaAs) in which the latticeconstant is mismatched and dislocations are generated (dislocationsdepicted as jagged lines in the bottom cell) but the subcell bandgap andtherefore subcell quantum efficiency and photocurrent are quiteadjustable.

The thick substrate prevents the dislocations from propagating into themiddle and top cells, as they would in standard metamorphic growths. Thephotocurrent may be easily matched and should lead to a net efficiencygain due to the extra voltage added by the bottom cell despite thepresence of the dislocations. The dislocations do degrade the bottomcell photovoltage, but the idea is to limit this degradation to bottomcell 16, FIG. 7 and not allow the dislocations to degrade other subcellson the opposite side of the substrate.

Although specific features of the invention are shown in some drawingsand not in others, this is for convenience only as each feature may becombined with any or all of the other features in accordance with theinvention. The words “including”, “comprising”, “having”, and “with” asused herein are to be interpreted broadly and comprehensively and arenot limited to any physical interconnection. Moreover, any embodimentsdisclosed in the subject application are not to be taken as the onlypossible embodiments.

In addition, any amendment presented during the prosecution of thepatent application for this patent is not a disclaimer of any claimelement presented in the application as filed: those skilled in the artcannot reasonably be expected to draft a claim that would literallyencompass all possible equivalents, many equivalents will beunforeseeable at the time of the amendment and are beyond a fairinterpretation of what is to be surrendered (if anything), the rationaleunderlying the amendment may bear no more than a tangential relation tomany equivalents, and/or there are many other reasons the applicant cannot be expected to describe certain insubstantial substitutes for anyclaim element amended.

Other embodiments will occur to those skilled in the art and are withinthe following claims.

1-20. (canceled)
 21. A semiconductor device comprising: a substratehaving a first side and a second side opposite from the first side; alower bandgap subcell on the first side of the substrate, the lowerbandgap subcell comprising a bottom subcell and grading layers, thebottom subcell and the grading layers comprising non-phosphorousmaterials and a first periodic table group V material; and a pluralityof higher bandgap subcells on the second side of the substrate, theplurality of higher bandgap subcells comprising a second periodic tablegroup V material, wherein the lower bandgap subcell has a lower bandgapthan a bandgap of the substrate, and the plurality of higher bandgapsubcells have higher bandgaps than the bandgap of the substrate.
 22. Thesemiconductor device of claim 21, wherein the substrate comprises adoped N-type GaAs substrate having a doping impurity concentration ofless than 5×10¹⁷ cm⁻³.
 23. The semiconductor device of claim 21, whereinthe grading layers comprise In_(x)(Al_(y)Ga_(1-y))_(1-x)As.
 24. Thesemiconductor device of claim 21, wherein the bottom subcell comprisesIn_(x)Ga_(1-x)As.
 25. The semiconductor device of claim 21, wherein theplurality of higher bandgap subcells comprise a first higher bandgapsubcell, the first higher bandgap subcell comprising a N-on-P cell, theN-on-P cell comprising In_(x)Ga_(1-x)As or GaAs.
 26. The semiconductordevice of claim 25, wherein the plurality of higher bandgap subcellscomprise a second higher bandgap subcell, the second higher bandgapsubcell comprising In_(x)Ga_(1-x)P or In_(x)(Al_(y)Ga_(1-y))_(1-x)P. 27.The semiconductor device of claim 21, wherein the bottom subcellcomprises a contact cap.
 28. The semiconductor device of claim 21,wherein the first periodic table group V material comprises a periodictable group V material of the substrate.
 29. The semiconductor device ofclaim 21, wherein the first periodic table group V material comprisesthe same periodic table group V material as the second periodic tablegroup V material.
 30. The semiconductor device of claim 21, wherein thefirst periodic table group V material comprises a different periodictable group V material than the second periodic table group V material.31. The semiconductor device of claim 21, wherein the lower bandgapsubcell is not lattice-matched to the substrate, and the plurality ofhigher bandgap subcells are lattice-matched to the substrate.
 32. Thesemiconductor device of claim 21, wherein a first higher bandgap subcellof the plurality of higher bandgap subcells has a different bandgap thana second higher bandgap subcell of the plurality of higher bandgapsubcells.
 33. The semiconductor device of claim 21, further comprising aprotective layer on the lower bandgap subcell.
 34. The semiconductordevice of claim 33, wherein the protective layer comprises siliconnitride.
 35. The semiconductor device of claim 21, in combination withan epitaxial reactor glovebox configured to flip the substrate beforegrowth of one of the lower bandgap subcell and the plurality of higherbandgap subcells on the substrate and after growth of one of the lowerbandgap subcell and the plurality of higher bandgap subcells on thesubstrate.